1. Field of the Invention
The present invention relates generally to sputtering systems and more specifically to an improved cylindrical magnetron.
2. Related Art
Sputtering is the process most often used for large area commercial coating applications, such as the application of thermal control coatings to architectural and automobile glazings. In this process, the substrates to be coated are passed through a series of in-line vacuum chambers isolated from one another by vacuum locks. Over the years, the magnetron used in these coaters has evolved from a planar to a rotating cylindrical design.
The rotating magnetrons while solving some problems produced others. These problems include new arcing phenomena, which are particularly troublesome in the DC or AC reactive sputtering of dielectric or insulating materials such as silicon dioxide, aluminum oxide and zirconium oxide. Insulating materials like silicon dioxide are particularly useful to form high quality, precision optical coatings such as multilayer, antireflection coatings and multilayer, enhanced aluminum reflectors, but are particularly difficult to sputter evenly and consistently. This is generally due to arcing that occurs at the ends of the magnetron due to buildup or condensation of the sputtered insulating materials.
A cylindrical magnetron generally comprises a rotating target tube having the material to be sputtered onto a substrate to be coated. Within the target tube is a ‘racetrack’ shaped magnet that is part of a system that excites ions to bombard the tube and sputter off atoms, which in turn coat the substrate. The racetrack shaped magnet has ‘turnarounds’ at both ends of the target tube. The turnarounds possess two significantly different properties than at the center area of the magnet assembly: 1) A relatively greater magnetic field strength and 2) a greater unit area at the target surface (roughly 3:2) being influenced by the magnetic field. Thus the sputtering material of the target tube sputters more rapidly near the turnaround. This has two noteworthy consequences. First, because the sputtering rate is higher in that area, if the article to be coated is located directly under or sufficiently near the turnaround, it will have a thicker coating deposited on it. Second, the target tube will wear much more rapidly at the turnaround area, and much of the central area of the target tube will go to waste if the tube is changed when the turnaround area has worn thin.
Additionally, to date, there always exists a portion of the target tube, at the ends of the target tube that is out of the effective range (‘sputtering zone’) of the magnetic field created by the racetrack. This portion out of the sputtering zone, yet not within the endblock will be referred to as ‘the unsputtered ends.’ The sputtering zone is self-cleaning because material is constantly being sputtered off, but the unsputtered ends are not self cleaning. In fact, material sputtered from the sputtering zone, in addition to coating the substrate, also coats other surfaces within the reactive chamber of the magnetron including the unsputtered ends. The unwanted coating is referred to as condensate.
When certain dielectrics are being sputtered and coat the unsputtered ends, arcing may occur. As dielectric films accumulate on the unsputtered ends, charge can build up rapidly and an arc may be produced when the dielectric film breaks down under the high electrical field produced by the charge accumulation across the film. The higher the dielectric constant of the film, the more likely arcing is to occur.
Arcing results in a non uniform coating of the substrate, and is therefore detrimental to cost effective operation, as any article being coated when an arc occurs will most likely be defective. For instance, the article may be contaminated by debris resulting from the arc, or it may have an area with incorrect film thickness caused by temporary disruption of the discharge conditions. Furthermore, the occurrence of arcs increases with operating time, and eventually reaches a level which requires that the system be shut down for cleaning and maintenance.
Many different approaches have been developed to minimize arcing and the consequences of arcing.
U.S. Pat. No. 5,108,574 to Kirs et al., hereby incorporated by this reference in its entirety, utilizes a shield to cover the unsputtered ends in order to minimize arcing. This type of dark space shield prevents the re-deposition of condensate on the target ends or backside. The idea in this patent is to provide a cylindrical enclosure that surrounds the sputter target and has an opening in the region where the sputtering occurs. In essence, a dark space shield that prevents re-deposition of condensate or ignition of plasma at or near the target ends or backside.
The cathode dark space is the darker region of the plasma near the cathode surface where most of the potential drop in a diode discharge occurs. A dark space shield is a grounded surface which is placed at less than a dark space distance from the cathode in order to prevent the establishment of a discharge in the region between the two surfaces. The dark space distance is proportional to the mean free path in the gas and thus the level of vacuum.
This patent applies to targets, used for research and development purposes, that are about 1 foot long and 3″ in diameter. Although the shielding may perform well on a small research system, scale-up to a production coater has proven to be difficult and more importantly, the problems associated with condensate adhering to the shield are not addressed; namely arcing and condensate falling onto the substrates.
U.S. Pat. No. 5,213,672 to Hartig et al., hereby incorporated by this reference in its entirety, utilizes an improved shield that can be implemented on large scale magnetrons.
U.S. Pat. No. 5,364,518 to Hartig et al., hereby incorporated by this reference in its entirety, manipulates the turnarounds of the racetrack shape magnet to minimize the Gaussian field in the turnarounds. This improves target utilization but runs the risk of losing electron confinement thereby potentially reducing the effectiveness of the magnetic array.
U.S. Pat. No. 5,527,439 to Sieck et al., hereby incorporated by this reference in its entirety, electrically floats the end shields so arcing cannot occur between the end shield and the target, incorporates grooves into the outer surface of the end shield that limit the damage an arc can cause when the condensate deposits on the outer surfaces of the end shield initiate an arc, and uses a notched area in the end shield that provides better shielding against redeposition of condensate.
U.S. Pat. No. 5,725,746 to Dickey et al., hereby incorporated by this reference in its entirety, utilizes a cylindrical region on each end of the cathode body which has a surface of a collar material different from the target material. The cylindrical region extends into the sputtering zone typically for a distance of about two inches. The collar material is sputtered as the target material is sputtered, but typically at a lower rate. The sputtered collar material forms films having poor insulating properties. These films deposit on the unsputtered cathode ends, dark space shielding and support structures in preference to the material sputtered from the target. Electrical leakage through these poorly insulating films reduces charge build-up and arcing.
U.S. Pat. No. 5,853,816 to Vanderstraeten, hereby incorporated by this reference in its entirety, incorporates a simple and straightforward approach . . . put more material where it is needed, at the ends of the target tube. This is cost effective for plasma-sprayed targets and results in high utilization for the cathode approaching roughly 90%+. However, it is cost prohibitive when using most other targets due to machining and material costs. Additionally, the thicker than normal area at the target ends mechanically interferes with most standard end shielding in use today and therefore provisions such as modifications to the end shield must be made. Finally, material thickness is ultimately limited by the physics of magnetic field strength diminishing with distance; when material thickness is too great, the magnetic field at the target surface becomes too weak and electron confinement is degraded and eventually lost all together
All of the aforementioned approaches fail to solve the underlying cause of the problem.